1. Field of the Invention
The present invention relates generally to the field of thermal writing of high density data media, and more particularly to the specific composition and methods of forming high density data recording products for use in near-field optical and electron beam writing.
2. Description of the Related Art
Phase-change media are a widely available means for providing high density data storage, and such media may include CD-RW, DVD-RAM, and DVD-RW formats, among others. In this type of media, data is stored at a particular location, typically micron sized, and storage and erasure occurs based on the microstructure of the target region. Microstructures are either crystalline or amorphous. Bit writing in crystalline media requires melting the desired region and then rapidly quenching the region to a glassy state. Bit erasure involves transitioning from this glassy state using relatively slow, gentle heating to produce recrystallization. Writing and erasure processes therefore require depositing significant amounts of energy into the phase change medium, also referred to as the phase change layer or data layer, which in the past has typically been a ternary compound of germanium, antimony, and tellurium, GeSbTe, also known as GST.
In this setting, ablation of the phase change medium or chemical changes introduced to the medium can be highly undesirable, and may introduce imperfections in the media and/or impede the ability to write and erase data bits on the media. Preventing unwanted ablation or chemical changes in the past has entailed cladding the GST layer between thick films of amorphous zinc sulfide-silicon oxide (ZnS—SiO2) dielectric material. The cladding layers tend to prevent ablation and chemical changes to the phase change medium and are compatible with the recording process due to their transparency to visible light. Additionally, the cladding layers offer significant resistance to heat conducted from the low melting temperature GST layer. Such an optical stack may also include an aluminum (Al) or gold (Au) layer which acts as a mirror and can provide a high conductance heat sink for the other layers. A typical ZnS—SiO2/GST/ZnS—SiO2/Al stack may be embedded in polycarbonate for durability purposes and ease of use by end users or consumers.
More recent memory designs have begun to employ near-field optics or electron beams in thermal writing. See, for example, U.S. Pat. No. 5,557,596, “Ultra High Density Storage Device,” issued Sep. 17, 1996 to inventor Gary A Gibson. The '596 patent provides for a plurality of electron emitters generating beams of electrons to information storage media areas on a movable platform to store and retrieve information. A micro mover, based on micro electro mechanical systems (MEMS) technology, moves the platform relative to the electron emitters to enable parallel communications with selected storage media areas on the platform. In the '596 patent, the data storage medium includes a diode whose top layer is a phase-change material that can be reversibly changed between crystalline and amorphous states (or between two crystalline states with different electrical properties). Data is written using an electron beam to locally effect a change of state in the phase-change layer. Bits are detected by interrogating a bit with an electron beam while monitoring the current induced in the diode. This induced current depends upon the local state of the phase-change layer in the interrogated region.
In near-field optical and electron beam thermal writing systems, the aforementioned thick stack (ZnS—SiO2/GST/ZnS—SiO2/Al or ZnS—SiO2/GST/ZnS—SiO2) cannot be used for various reasons. First, near-field writing requires that the optical probe pass within much less than one wavelength of the media surface, which is generally incompatible with typical ZnS—SiO2 cladding thicknesses employed, as well as being incompatible with the polycarbonate used to cover the media. In electron beam thermal writing, electron beams typically cannot penetrate through relatively thick cladding layers unless extremely high beam energies are employed, which is impractical. Furthermore, thick insulating cladding layers tend to charge and deflect the electron beam, which is also undesirable.
The problems of ablation and chemical modification of the data layer remain, however, in the presence of near-field optical and electron beam thermal recording schemes.
It would be advantageous to provide a design having the advantages associated with inhibiting ablation and chemical modification in high density and ultra high density media, while at the same time enabling near-field optical and electron beam thermal writing and erasure of said media in a relatively efficient manner.